The present invention relates to an image forming apparatus for forming a latent image on an image carrier via an exposure, developing the latent image by a developing agent into a visible image, and transferring the developed visible image to a transfer sheet.
In copying machines and laser printers which utilize the electrophotographic recording process, a photosensitive drum, which serves as an image carrier, is repeatedly used. For this reason, a photosensitive drum cleaning means for removing residual developing agent particles left on the photosensitive drum must be disposed between a transferring means for transferring a visible image to a transfer sheet and a latent image forming means for forming a latent image on the photosensitive drum by charging.
One conventional laser printer has the construction shown in FIG. 1. A photosensitive drum 2 disposed at the center in a housing 1 is uniformly charged with a positive polarity by a charger 3. A laser beam 5 scanned by a scanner 4 is focused by an F.theta. lens 6 on the photosensitive drum 2. The laser beam 5, which is intermittently emitted upon the ON/OFF operation of the power supply, exposes the surface of the photosensitive drum 2 to selectively discharge the photosensitive drum 2, thereby forming a latent image thereon. The latent image on the photosensitive drum 2 is developed with a developing agent, by a developing unit 7 having a developing roller to which a proper bias voltage is applied. The developed visible image is transferred by a transferring charger 8 to a transfer sheet P. The transferred image is then fixed by a fixer 9. The residual developing agent particles which are not transferred to the sheet P but are left on the photosensitive drum 2 are removed by a cleaning blade 11 of a cleaning unit 10. The cleaned photosensitive drum 2 is discharged by a discharging lamp 12. The photosensitive drum 2 is then uniformly re-charged with the positive polarity by the charger 3. The above operation is then repeated.
A plurality of transfer sheets P are set in a supply cassette 13 and are picked up, one by one, by a pick-up roller 14. The leading end of each picked-up transfer sheet P is adjusted by an aligning roller 15, to correct the ramp of the sheet, and the sheet is fed into a transfer section defined between the transferring charger 8 and the photosensitive drum 2 at an image forming timing of the photosensitive drum 2. Upon operation of the transferring charger 8, the transfer sheet P having the visible image thereon is separated from the photosensitive drum 2 and is supplied to the fixer 9. Thereafter, the fixed transfer sheet P having the visible image thereon is ejected into a tray 18 through an exhaust roller 17. The exhaust tray 18 is disposed at that side of the housing 1 which opposes the side at which the supply cassette 13 is mounted.
FIGS. 2, 3 and 4 respectively show conventional processes wherein the photosensitive drum is repeatedly used. A cycle of charging (1), exposure (2), development (3), transfer (4), cleaning (5), discharging (6), and charging (1), as shown in FIG. 2, indicates a normal development process (wherein a developing agent having a polarity opposite to that of the charged carriers is attracted to a nonexposed portion of the photosensitive drum) wherein the photosensitive drum is repeatedly used. On the other hand, a cycle of charging (1), exposure (2), development (3), transfer (4), cleaning (5), discharging (6) and charging (1), as shown in FIG. 3, indicates a reverse developing process (wherein a bias voltage is applied to the developing roller, and one-component developing agent particles are charged and are attracted to an exposed portion of the photosensitive drum) wherein the photosensitive drum is repeatedly used.
As is apparent from the conventional processes described above, to repeatedly use the photosensitive drum, the residual developing agent particles must be removed from the photosensitive drum after each transfer operation is completed. In the normal development process shown in FIG. 2, one may assume that cleaning (5) is omitted and that the photosensitive drum is immediately discharged after transfer (i.e., that a cycle is performed in an order of charging (1'), exposure (2'), development (3), transfer (4), discharging (6') and charging (1')). In this cycle, excluding the cleaning operation, the residual developing agent on the photosensitive drum is charged during charging (1'), and is exposed by exposure (2'). Therefore, the residual developing agent particles will not be removed from the photosensitive drum during development (3), since the residual potential of the photosensitive drum becomes substantially the same as the developing bias voltage. The residual developing agent particles are transferred to the transfer sheet P during transfer (4). Therefore, the residual developing agent particles cause background fog or scumming, resulting in a poor image.
Even in the reverse development process using one-component developing agent particles, one may assume that cleaning (5) is omitted and that transfer is immediately followed by discharging, i.e., that a cycle is performed in an order of charging (1'), exposure (2'), development (3), transfer (4), discharging (6') and charging (1'). In the process excluding cleaning (5), the charged particles left on the charged portion of the latent image after exposure (2') cannot be removed, since the developing bias voltage V becomes substantially the same as the surface potential V0 of the charged portion of the photosensitive drum during development (3). In the same manner as in the cycle shown in FIG. 2, the residual developing agent particles are transferred to a transfer sheet during transfer (4), thus resulting in a poor image. Therefore, cleaning of the photosensitive drum must inevitably be performed between transfer (4) and charging (1), in the processes shown in FIGS. 2 and 3, wherein the photosensitive drum is repeatedly used.
However, to clean the photosensitive drum, the requirement for a cleaning unit increases the internal space required for the housing, so that the recording apparatus (as a whole) would be large in size. Second, a mechanical stress acts on the photosensitive drum since a cleaning member such as a cleaning blade must be brought into sliding contact with the photosensitive drum. The photosensitive drum is susceptible to damage and to the formation of a film of residual charged particles, thus degrading the image quality.
To solve the above problem, a conventional process has been proposed, wherein a photosensitive drum is rotated twice for one image forming cycle, in such a way that a developing bias voltage changes during the second revolution of the photosensitive drum, to employ a developing unit as a cleaning unit, which developing unit is used for development during the first revolution of the photosensitive drum. FIG. 4 shows this process. The photosensitive drum is rotated for the first time, becoming operable in an order of charging (1), exposure (2), development (3), transfer (4) and discharging (5). The photosensitive drum is then rotated for the second time, to perform a cleaning operation in an order of discharging (5), cleaning (6) and charging (1). Referring to FIG. 4, the photosensitive drum is uniformly charged with a positive polarity during the charging (1) stage, and has a surface potential V0. The photosensitive drum is then exposed by exposure (2) so that a latent image is formed thereon. By development (3), a developing roller which is so biased as to have a potential which is equal to or slightly higher than the potential of the residual potential of the exposed portion of the photosensitive drum causes the developing agent having a polarity opposite to that of the charged carriers on the latent image to be attracted to the charged portion of the latent image. During transfer (4), the developed visible image on the photosensitive drum is transferred by a transferring charger to a transfer sheet. By discharging (5), the photosensitive drum is electrically discharged by a discharging lamp. Thus, the photosensitive drum is rotated by one revolution. Thereafter, the developing bias voltage V is so set as to fall within a range of 0&lt;V&lt;V0. Under such conditions, the developing roller serves as a cleaning means, to remove the residual developing agent particles from the photosensitive drum while the photosensitive drum completes the second revolution. In this manner, one recording cycle is completed by two revolutions of the photosensitive drum.
However, in this process for allowing repeated use of the photosensitive drum, the circumferential length of the photosensitive drum must be longer than that of an image recorded by one cycle. One may assume that the circumferential length is shorter than that of the image. When the leading end of the image formed on the photosensitive drum reaches a position where it opposes the developing roller, the trailing end thereof is still subjected to development. As a result, the developing roller cannot serve as the cleaning means, and the residual developing agent particles on the drum portion carrying the leading end of the image are left on the photosensitive drum and cannot be removed therefrom. Therefore, according to this process, the circumferential length of the photosensitive drum, hence, its outer diameter, must be increased. In addition to this disadvantage, since one of every two revolutions is used for cleaning, the utilization efficiency of the photosensitive drum is reduced to 50%. The recording rate is thus decreased, and two bias power supplies are required to change the bias voltage applied to the developing roller.
In the image forming apparatus which employs the reverse development process and uses a two-component developing agent, the cleaning of the photosensitive drum is performed between transfer and charging, in the same manner as in FIG. 1.
In conventional copying machines and laser printers which use the electrophotographic recording process, removal of the residual developing agent particles left on the photosensitive drum as an image carrier is performed between transfer and latent image formation, by means of charging and exposure, with respect to the photosensitive drum. Therefore, the developing means for applying the developing agent to the photosensitive drum is so located as to be spaced apart from the cleaning means, with the result that the inside of the housing tends to become contaminated, necessitating countermeasures for preventing the respective components from becoming contaminated.